Chapter 2 – Orientation for the Bio-Curious  23

differences equivalent to the typical thermal energies of a few water molecules can

make the difference between phase transition occurring or not. In other words, LLPS

droplets are relatively unstable.

A minority of scientists consider viruses to be a minimally sized unit of life; the smallest

known viruses having an effective diameter of ~20 nm (see Figure 2.1 for a typical virus image,

as well as various cellular features). Viruses are indeed self-​contained structures physically

enclosing biomolecules. They consist of a protein coat called a “capsid” that encloses a simple

viral genetic code of a nucleic acid (either of DNA or RNA depending on the virus type).

However, viruses can only replicate by utilizing some of the extra genetic machinery of a host

FIGURE 2.1  The architecture of biological structures. A range of typical cellular structures,

in addition to viruses. (a) Rodlike maize mosaic viruses, (b) obtained using negative staining

followed by transmission electron microscopy (TEM) (see Chapter 5); (c) mitochondria from

guinea pig pancreas cells, (d) TEM of nucleus with endoplasmic reticulum (ER), (e) phase con­

trast image of a human cheek cell. (a: Adapted from Cell Image Library, University of California at

San Diego, CIL:12417 c: Courtesy of G.E. Palade, CIL:37198; d: Courtesy of D. Fawcett, CIL:11045;

e: Adapted from CIL:12594.)